The present invention relates to a method of securing prefabricated lagging components to a metal surface and to a prefabricated lagging component for use in carrying out the method.
Metal surfaces have to be lagged, e.g. in the case of metal tubes at high temperatures in heating and heat-treatment furnaces, in order to protect the tubes and keep them away from the action of flames. The same problem occurs in connection with furnace flues or ducts conveying products or ventilation shafts and the like.
There are a number of methods of lagging the surfaces to be protected, but all are substantially based on two processes. In the first process the lagging material, e.g. a refractory material, is positioned on the metal surface before being shaped. It is cast or tamped on to the surface in a formwork made for the purpose and secured to the surface by a large number of studs on the surface to be protected. Before being used, the resulting lagging must be carefully and uniformly dried so that the material does not contain regions at different tensions or having different structures such as to produce faults such as cracks and/or blisters, since such faults may reduce the efficiency of the lagging and even cause damage resulting in the disappearance of the lagging over relatively large areas of surface, thus requiring repairs which are always very difficult and expensive and interfere with the operation of the device in question. The disadvantages of the method are that it requires closely-packed securing studs, operator specialized in the laying of lagging material, a long drying process and, possibly, difficult and expensive repairs. The reliability of the lagging is not absolute and the surface protection depends not only on the physical properties of the material used but also on the quality of the positioning work. Finally, the close-packed securing means make it practically impossible to insert an insulating lining between the metal surface and the refractory lining.
The second process consists in using prefabricated lagging components (conventionally called shells) which are positioned on the surface to be protected and secured bu fitting together or by metal attachments which are fixed or previously placed on the surface to be protected. The method has a disadvantage in that the lagging components, when fitted together, are fragile at the places where they fit together, are relatively expensive per linear meter, and can be secured only to vertical surfaces. When the components are secured by metal attachments, laborious work is required for securing the attachments to the surface to be protected. In some cases each component has to be adjusted and in other cases it has to be preassembled. In addition, prefabricated components frequently break down and the resulting lagging is also expensive per linear meter.